1. Field of the Invention
The present invention relates to a driving apparatus for a cold cathode fluorescent lamp (CCFL). More particularly, the present invention relates to a driving apparatus for a CCFL capable of easily providing a high output required to start the lamp.
2. Description of the Related Art
A cold cathode fluorescent lamp (CCFL) has fluorescent material coated onto its interior wall of a glass tube. The CCFL lamp has electrodes attached to both ends of the tube, where tens of Torr of gas mixtures and a predetermined amount of mercury are sealed. The CCFL bears plenty of advantages such as high brightness, high color rendering, low power consumption of 2.0 W to 5.0 W, long useful life of 15,000 to 60,000 hr in a rated lamp current. Therefore it is well-utilized as a lighting device in an LCD back-light.
However, the CCFL exhibits large impedance if current is not applied and smaller impedance once the lamp is ignited. Therefore, to start the lamp requires high electric field energy of at least 1300V and about 800V is required once the lamp is ignited.
Thus, the driving apparatus for the CCFL should provide appropriate electric field energy in accordance with a lamp-on/lamp-out condition.
FIG. 1 is a circuit diagram illustrating a driving apparatus for a CCFL. Referring to FIG. 1, the driving apparatus for the CCFL 10 includes: an oscillator 11 for providing a reference signal; a pulse width modulation (PWM) driver 12 for dividing the reference signal outputted from the oscillator 11 by a predetermined rate, and comparing the divided signal with a PWM dimming control signal applied from the outside to output a PWM switching control signal having an on/off duty rate adjusted; an inverter 13 for switching on and off in response to the switching control signal outputted from the PWM driver and inverting direct current into alternating current of a predetermined frequency; and a resonance circuit 14 for outputting the alternating current of a predetermined frequency outputted from the inverter 13 to the lamp 10.
The inverter 13 includes two switching devices SW1, SW2 of a half bridge structure connected in series between a current input terminal and a ground. The switching devices SW1, SW2 switch on/off alternately to output the alternating current of a predetermined frequency. The alternating current outputted from the inverter 13 is applied to the lamp 10 through the resonance circuit 14.
A PWM controller 12 divides the reference signal of a predetermined frequency outputted from the oscillator 11 by ½, and adjusts an on/off duty rate of the divided signal in response to a PWM control signal VPWM to control brightness of the lamp 10.
In the driving apparatus for the CCFL of this configuration, high electric field energy required to start the lamp can be attained by controlling only an on/off duty rate of the switching devices SW1, SW2 through the PWM controller 12, with a switching speed of the inverter 13 fixed.
FIG. 2 is a timing diagram for explaining a driving function of a conventional CCFL in case of starting the lamp, in which (a) and (b) are switching control signals applied to the switching devices SW1, SW2, respectively, and (c) is an output current applied to the lamp 10 in case where the switching control signal is applied as in (a) and (b). Referring to FIG. 2(a) and (b), conventionally, if the lamp needs to be ignited, the switching control signal of a maximum duty rate is applied to the inverter until the lamp 10 turns to an ignited condition from an extinguished condition. After the lamp 10 is ignited, the duty rate is adjusted in accordance with a designated brightness.
However, in case where the inverter 13 of a half bridge structure is employed as described above, two switching devices SW1, SW2 should not be on at the same time and thus the maximum duty rate is 50% or less. This renders it impossible to satisfy the voltage required to start the lamp. Also, in case where the voltage for starting the lamp cannot be sufficiently boosted, the lamp operates unstably like flickering.